This invention is in the field of broad band active circuits and more specifically broad band mixers.
A mixer is a threeport device, having a signal (RF) input, local oscillator (LO) input, and an intermediate-frequency (IF) output. An ideal mixer simply translates the input signal, leaving everything else unchanged. In actual use, harmonic and product signals are generated in the mixing process. In conventional mixer arrangements, these signals may undergo additional mixing and cause variations in the mixer's conversion-loss flatness as a function of frequency. Mixers that operate over a broad frequency range at one input and a narrow frequency range at another output and use reactive filters at these ports are especially susceptible to the above problem. This is the result of impedance mismatches between the filters and the mixer terminals.
Prior methods of correcting for this problem usually add a resistive attenuator in the signal path to better match impedances and reduce reflected signals. Obviously, this technique degrades mixer sensitivity by the amount of the signal attenuation added.
U.S. Pat. No. 3,737,784 to Dostis describes a system wherein broadband impedance matching is provided at mixer terminals by using directional filters. However, the approach taken by Dostis does not solve some problems. For example, if one terminal is connected to a signal input that varies from DC to some upper limit the described directional filter cannot be used. Also, there are certain instances when a directional filter cannot produce the required bandpass width and shape.